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Thursday, 23 February 2017

Co-ordination Compounds - I

Co-ordination compounds:
A coordination compound contains a central metal atom or ion surrounded by number of oppositely charged ions or neutral molecules. These ions or molecules re bonded to the metal atom or ion by a coordinate bond.Example: K4[Fe(CN)6]They do not dissociate into simple ions when dissolved in water.Double saltWhen two salts in stoichiometric ratio are crystallised together from their saturated solution they are called double saltsExample:FeSO4.(NH4)2SO4.6H2O (Mohr’s salt)They dissociate into simple ions when dissolved in water.Coordination entity:A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules.Example: In K4[Fe(CN)6], [Fe(CN)6]4- represents coordination entity.Central atom or ion:In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion.Example: In K4[Fe(CN)6], Fe²+ is the central metal ion.Ligands:A molecule, ion or group that is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond is called ligand.It may be neutral, positively or negatively charged.Examples: H2O, CN-,NO+etc.Donor atom:An atom of the ligand attached directly to the metal is called the donor atom.Example: In the complex K4[Fe(CN)6], CN is a donor atom.Coordination number:The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded.Example: In the complex K4[Fe(CN)6], the coordination number of Fe is 6.Coordination sphere:The central atom/ion and the ligands attached to it are enclosed in square bracket and are collectively termed as the coordination sphere.Example: In the complex K4[Fe(CN)6], [Fe(CN)6]4- is the coordination sphere.Counter ions:The ions present outside the coordination sphere are called counter ions.Example: In the complex K4[Fe(CN)6], K+ is the counter ion.Coordination polyhedron:The spatial arrangement of the ligand atoms which are directly attached to the central atom/ ion defines a coordination polyhedron about the central atom.The most common coordination polyhedra are octahedral, square planar and tetrahedral.Examples: [PtCl4]2- is square planar, Ni(CO)4 is tetrahedral while [Cu(NH3)6]3+ is octahedral.